Sheet metal is the preferred material of construction for the fabrication of ducts and other fittings used in heating and ventilation systems. For many of the fittings, a hole or ring must first be cut in a flat metal sheet prior to the fixation of a tubular or conical piping member thereover.
One method for cutting a hole in a sheet metal blank is by using a manually operated circle shear machine. An example of a typical manually operated circle shear machine is the model No. 298 offered by the Roper Whitney Co. of Rockford, Illinois. This machine includes two opposed cutter wheels which are clamped down on a top and bottom surface of the metal sheet. The cutter wheels are then power rotated to draw the metal sheet through the cutter wheels in a circular pattern to form a hole cut-out in a manner similar to a can opener spinning a can in which it is opening.
A principal disadvantage with this type of circle shear machine is that it cannot be feasibly implemented in existing automatic feed systems. Current state of the art automatic feed systems for sheet metal fabrication typically comprise a plurality of different in-line sheet metal working stations (e.g., a blanking machine, a forming machine, etc.), and are linked together by a belt or chain drive transport mechanism. The sheet metal is held stationary at each metal working station and then advanced to the next station via a conveyor. Implementation of the aforementioned circle shear machine would result in major complications in the handling of the metal sheet, since now the metal sheet must be rotated in the middle of the automated line in order to generate a hole cut-out by the circle shear machine. The handling difficulties would only be increased when a wider diameter circle cut is desired since this requires that the entire sheet metal section be moved through a wider circular path by the cutter wheels.
U.S. Pat. No. 4,397,207 (Isaao, 1983 ) teaches to provide a sheet metal cutting device which is mounted on a wheeled dolly. The device includes an adjustable trammel assembly which guides the dolly in an arc as a pair of powered cutting wheels work their way through the metal sheet. Isaac is directed towards cutting large radius curves and circles for forming conical upper portions of grain bins and like applications and is not well adapted for cutting smaller holes for use in heating and ventilation systems on a mass production scale. Also, it is not feasible to implement the cutting device of Isaao into existing automated sheet metal fabrication machines since the wheeled dolly must be permitted to move around the sheet metal to make a cut.
Another method for cutting holes in sheet metal sections is by using a circular die to punch a hole in the metal sheet. This method is particularly well suited for use in an automated line wherein a large run of sheet metal blanks having the same size hole is desired. However, this method becomes cost prohibitive for smaller production runs of sheet metal blanks and for production runs having different sized diameter holes, since a separate die is required for each different size diameter hole. Replacing dies is a labor intensive process which slows down productivity. Also, the use of dies in general means increased tooling costs. It is not uncommon for sheet metal fabricators to invest $20,000 to $40,000 for an adequate supply of dies. Moreover, as the dies are used, additional expense is incurred for periodic resharpening and replacement
Accordingly, there is a definite need in the art for an improved circle shear machine which is simple to operate and is easily implemented in current automatic feed sheet metal fabrication systems. There is also a need for such a circle shear machine which includes means for simple adjustment of the cutting hole diameter and is economical to operate in terms of tooling costs.